1. Field of Invention
This invention relates generally to the biological treatment of wastewater and particularly to a method and system for optimizing the operation of a trickling filter.
2. Description of the Prior Art
The biological treatment of wastewater for the removal of oxygen demanding carbon and nitrogen compounds has been in use for many decades, both in the United States and Europe. Although there are several types of systems available, the rotary biological filter and the trickling filter are the most common. The trickling filter is used primarily for reducing levels of BOD (biochemical oxygen demand) and TSS (total suspended solids) as well as the oxidation of ammonia to nitrates. Depending on the predominant ambient temperature at which the trickling filters are operated, some of the filters are covered or domed, while others are not. Examples of biological filters can be found in U.S. Pat. Nos. 2,642,394; 3,275,147; 3,596,767 and 4,486,310.
In order to better understand the current status of biological filters in the art, the following publications are of particular interest: Hawkes, H.A. "The Ecology of Waste Water Treatment" published by the MacMillan Co., New York, N.Y. 1963; Albertson, Orris E., "Slow Down That Trickling Filter!" WPCF Operations Forum, A WPCF publication for wastewater professionals, January, 1989, and West Germany, Ein Regelwerk der Abwassertechnischen Vereinigung (ATV) Arbeitblatt A135 Section 3.2.2, Tropfkorperbemessung, April 1983.
The conventional trickling filter utilizes a film of biomass fixed on a media to remove and aerobically convert organic matter to carbon dioxide, water and additional biomass and to oxidize ammonia to nitrates. The fixed media generally consists of rock, plastic or wood. The surface area of the media varies from 12-15 ft.sup.2 /ft.sup.3 for rock and wood to 27-45 ft.sup.2 /ft.sup.3 for plastic. New construction of trickling filters uses predominantly plastic modules as its media at depths of at least 5 ft. to higher than 40 ft.
Wastewater is distributed over the biomass fixed to media through an overhead rotary distributor having generally two to four nozzled arms or spreaders. This insures a relatively even distribution of wastewater over the fixed biomass and thereby produces a relatively constant loading throughout the filter area.
Microorganisms and other forms of biological life are the active agents for converting the organic carbon and nitrogen into environmentally acceptable products. As a result, a number of operating parameters affect the efficient operation of trickling filters. Some of these include temperature, organic loading, aeration, wastewater characteristics, filter depth and biomass thickness, to name but a few.
In current practice, it is not uncommon to utilize biomass thickness of between 0.10 to 0.30 inch (2.5-8 mm) as reported in the Albertson publication, mentioned above. Such biomass thicknesses can reduce the aerobic surface area by a factor of 20% or more. The aerobic biomass may be only 5-10% of the total biomass on the media. The excess or remaining biomass (90-95%) serves no useful purpose in organic removal. Instead, the excess accumulation of biomass produces numerous operating problems, such as (1) production of odors; (2) provides a haven for flies and snails; (3) reduces the aerobic surface area; (4) causes excess sloughing of biomass which in turn causes an imbalance to downstream processes and loss of aerobic biomass; and (5) discolors the filter effluent.
Although there are a number of factors which affect the performance of trickling filters, such factors generally affect, either directly or indirectly, the growth of its active agent, the biomass. Therefore, one can conclude that it would be highly beneficial to control the biomass thickness to minimum levels and thereby provide a biomass layer which is predominantly aerobic and eliminate, as much as possible, the underlayer of anaerobic biomass. In the past, the biomass thickness has been partially and often ineffectively controlled by inconsistent and intermittent dosing or flushing of the media. Since trickling filter distributors are normally driven by the hydraulic jetting action of its nozzled arms, any increase in wastewater flow also increases the speed at which the nozzled arms pass over the biomass. With such systems, the instantaneous dosing or flushing of the biomass is frustrated.
In 1983, West Germany reported in a publication identified above that the dosing rate or flushing intensity (SK value) of biomass was critical in improving the efficiency of trickling filters. SK may be defined generally as the depth of water discharged to the surface of the filter media per pass of a distributor arm. This SK (spulkraft) value is determined by the following formula: ##EQU1## Wherein "SK" represents the flushing intensity per pass in mm/pass, "q+r" represents the total average hydraulic flow in m.sup.3 /m.sup.2 /hr, "a" represents the number of distributor arms and "n" represents the rotational speed in rev./min.
Currently, SK values of between 2-10 mm/pass of an arm are common. Recently, (Albertson publication) SK valves of 50-150 mm/pass were found to be highly beneficial. Still more recently, Albertson determined that SK values up to 680/mm/pass have produced optimum results on strong (high BOD loadings) waste waters. However, this does not mean that still higher SK values would be more beneficial as, at some point, high SK values will impair the filter's performance even if the biomass thickness is near optimum. Therefore, based on the prior art, there appears to be no particular optimum SK value for any given trickling filter. This is primarily due to the wide range of operating variables which may exist for any given wastewater. It would also appear that the optimum SK value for biological treatment will vary substantially from the optimum SK value for flushing of excess biomass.